In recent years, a computer to plate system (CTP), in which image data can be directly recorded in a printing plate material, has been widely used in conjunction with the digitization of printing data. As a usable printing plate material for CTP, there are a printing plate material comprising an aluminum support such as a conventional PS plate, and a flexible printing plate material comprising a flexible resin film sheet and provided thereon, various functional layers.
Recently, in the commercial printing industry, there is a tendency that many kinds of prints are printed in small quantities, and an inexpensive printing plate material with high quality has been required in the market. As a conventional flexible printing plate material, there are a silver salt diffusion transfer type printing plate material as disclosed in Japanese Patent O.P.I. Publication No. 5-66564, in which a silver salt diffusion transfer type light sensitive layer is provided on a flexible sheet; an ablation type printing plate material as disclosed in Japanese Patent O.P.I. Publication Nos. 8-507727, 6-186750, 6-199064, 7-314934, 10-58636 and 10-244773 in which a hydrophilic layer and a lipophilic layer, one of which is an outermost layer, are provided on a flexible sheet where the outermost layer is ablated by laser exposure to prepare a printing plate; and a heat melt type printing plate material as disclosed in Japanese Patent O.P.I. Publication No. 2001-96710 in which a hydrophilic layer and a heat melt image formation layer are provided on a flexible sheet where a hydrophilic layer or a heat melt image formation layer is imagewise heated by laser exposure to heat and fix the image formation layer onto the hydrophilic layer.
On the other hand, in view of environmental consideration and so forth, a printing plate after writing image data (imagewise exposure) is mounted directly on an off-set press. Commonly known is a so-called on-press developing method in which, when on printing, dampening water is supplied to the printing plate material, whereby only the image formation layer at non-image portions is swollen or dissolved by the dampening water, and transferred to a printing paper (paper waste) at an initial stage of printing to remove (refer to Patent Documents 1 and 2). By providing the printing plate material capable of on-press printing, not only fresh dot shape and high quality images are obtained, but also a simple process in which a developing process is unnecessary to be used, and excellent environment suitability is attained.
However, the above printing plate material has problems in that initial ink receptivity and printing durability are degraded, since a hydrophilic layer and an image formation layer are weak in layer strength. A trial to solve the objective is made by adding a water-soluble resin or a thermoplastic resin into the hydrophilic layer and into the image formation layer (refer to Patent Document 3), but in the case of printing employing powders, the problem has not yet been solved, and development of a printing plate to improve the above objective has been strongly desired.
As to a thermal processless printing plate material, image visualization after image recording (hereinafter also referred to as exposure image visualization) is provided as one of the important desired characteristics. As a method for providing exposure image visualization, a method (refer to Patent Document 4, for example) is cited which employs a color fading property due to exposure of an infrared absorbing dye. However, when such a dye is added into an image formation layer, color difference between exposed portions and unexposed portions is increased to improve exposure image visualization, i.e. to increase an optical density at unexposed portions, resulting in problematic contamination of a printing press during on-press developing at unexposed portions, and it has been difficult for prior art to give sufficient printing durability and excellent exposure image visualization to the processless printing plate.
(Patent Document 1) Japanese Patent O.P.I. Publication No. 9-123387
(Patent Document 2) Japanese Patent O.P.I. Publication No. 9-123388
(Patent Document 3) Japanese Patent O.P.I. Publication No. 2000-238451
(Patent Document 4) Japanese Patent O.P.I. Publication No. 11-240270